The problem:
Color tags look simple but when you have multiple colors, than you don't remember the meaning for all of them. For example I remember that red tagged emails are important, and green ones financial emails, but I forgot what are the blue for.

The solution: Use icons instead of color tags OR use text tags.
For icon tags see the attached image:

2. Users & use cases

For example when I make my tax, I have to check my emails to see when money came in and when went out. For this I would make 2 icons (see image with pig icon) or simply I would create two text tag "money in" "money out" Than I would tag all the important emails accordingly and when I would need than I can find it better.

Another example: When I have to make an invoice for a client I have to check the exact sum in my proposal. I always spend a lot of time finding these emails. In this case a text tag would be more useful, like "proposal"

Another example and feature proposal: some email have different level of importance, and some of them has to be resolved in 2 days, we could have an icon or text tag with "date awareness" and could appear near the folder name, and you can see, that you have 3 important emails to resolve/answer today.